AbstractThe presence of chitinase was checked in three species ofHydrozoaby incubating γ‐chitin‐red (Hackmann and Goldberg, '64) with crude extracts of entire polyps or parts of them. When the substrate is lytically degraded it releases its soluble red dye the quantities of which were spectrophotometrically (λ = 510 nm) determined. Crude extracts of entire polyps ofHydra attenuataPall.,H. circumcinctaM. Sch., andPodocoryne carneaM. Sars all contained active chitinase. A more detailed localization which was studied inH. attenuatarevealed that the enzyme is confined to the endoderm of the body column and that it is absent in the ectoderm and in the tentacles which contain the stenotele nematocysts. Therefore, it is very unlikely that enzymatic action of chitinase aids the nematocyst in boring holes in the cuticule of the